The present invention relates generally to the field of duplex communications, and more particularly to systems wherein a master station transmits to and receives from a plurality of remote stations over a single channel.
There are many types of systems wherein one master unit communicates with a number of remote units using a two-way flow of data over a single communication channel. Examples of network systems sharing a common media are LAN (Local Area Networks), satellite communication, cable TV or wireless RF (Radio Frequency) transmission paging or mobile cellular phone systems. In these cases the master station broadcasts outbound signals to all remote stations simultaneously while the communication in the opposite direction i.e., from remote stations to the master, is done through multiple inbound channels time-shared between all remote units. In such systems each remote unit has a unique assigned address or identifier and the master is capable of polling each of them, broadcasting a polling signal containing the address of a target remote unit. Upon receiving the polling signal the addressed remote unit is due to respond even though it has no data to send however, permitting the master to keep track of the fleet of remote stations thus, for example, being updated with a status of the polled remote unit.
As the number of remote stations increases and they are more and more used the time left, to each of them, of the common inbound communication channel, i.e., from slaves to master, may dramatically be impacted up to a point where collisions are unavoidable or no communication at all becomes possible, for some period of time, between the master and some remote stations just because there were not enough inbound channels left. Then, in such systems, it is of the utmost importance that the common communication channel be used as efficiently as possible so as it can be shared between many users.
Also, it must be pointed out that if the single broadcast outbound communication channel, from the master to the slaves, can be adapted in many ways to make sure that all remote stations receive polling signals of a sufficient amplitude and power so as they can be safely interpreted by the receiving part of the remote units, generally mobile or portable units, the opposite is hardly possible. Quality of the broadcasting may be improved, for example, by increasing the power of the master transmission device or by equipping the area that must covered by the communication network with enough regenerators or relays. On the contrary, because they are generally portable units, the outbound power emitted from each remote unit is drastically controlled, for many obvious reasons, such as the necessity of limiting the power consumption from a battery or the level of the RF signal which must stay safe for the human being handling the device.
As a consequence the quality of the communication is intrinsically xe2x80x9casymmetricalxe2x80x9d. That is, the numerous inbound channels are much prone to errors than the single broadcasting outbound channel which can be improved at both ends by upgrading the characteristics the transmitting source (or multiplying them) and improving the level of reception of the receiver part of the remote units without being constrained by the above limiting factors dealing with the capacity of a battery or the harmless level of RF power which can be emitted from a portable device.
It is therefore one purpose of the present invention to provide a polling method and system for two-way communication between a multiplicity of remote units and a master station over a common media in which the broadcast outbound channel is more reliable than the individual inbound channels.
The invention discloses in a duplex communications system comprising a master station which controls access by a multiplicity of remote terminals to a broadcasting channel, said channel being for transmitting and receiving signals to and from said controlled remote terminals, a method of polling from said master station all said remote terminals, comprising the steps of:
broadcasting from a master station a polling command destined to a particular remote terminal, the polling command including a unique identifier and a status of the particular remote terminal which is stored in the master station;
receiving in all the remote terminals the polling command which is decoded so that only one of the remote terminals, recognizing its identifier, performs a self selection;
comparing, in the self selecting remote terminal, the status read in the polling command versus the actual current status of the self selecting remote terminal;
if no difference is observed between the current status of the terminal and the status in the polling command, forwarding to the master station, from the self selecting remote terminal, a response to the polling command including a report that no difference is observed between the current status of the terminal and said status of said polling command;
if a difference is observed between the current status of the terminal and the status in the polling command , forwarding to the master station, from the self selecting remote terminal, a response to the polling command including a report of the differences observed between the current status of the terminal and the status of the polling command; and,
updating in the master station the stored status of the answering terminal with the status corresponding to the reporting received in the response from the terminal.
The method of the invention takes advantage of an asymmetrical broadcasting communications channel in which the outbound channel, from a master station to a multiplicity of remote terminals, is more reliable than the individual inbound channels. Then, the remote terminals have only to respond with the changes, if any, observed versus the status broadcasted from the master station. This scheme permits limiting the size of the responses thus, the time needed to transmit them and serves two purposes. Firstly, this increases the probability for a response of not being altered over the lesser reliable channel. Secondly, it improves channel efficiency in freeing bandwidth so that more users may share simultaneously a common scarce transmission medium. Moreover, this scheme allows to implement most of the error recovery mechanism in the remote terminals which, being periodically updated from the master station, know exactly what they have to re-send in case of failure. This is achieved without requiring any assistance from the master station whose processing capability remains completely available for managing the network.